Synergistic combinations of clarithromycin with doxycycline or minocycline reduce the emergence of antimicrobial resistance in Rhodococcus equi

Background The alarming increase in rifampin and macrolide resistance among Rhodococcus equi isolates highlights the need to identify alternative therapeutic options that can effectively control rhodococcosis in foals while limiting the further development of drug resistance. Objectives To evaluate bacterial killing, antibiotic synergism and mutant prevention concentrations (MPCs) of clarithromycin alone and in combination with doxycycline, minocycline or rifampin against clinical isolates of R equi. Study design In vitro experiments. Methods Bacterial time-kill, fractional inhibitory concentration (checkerboard) and mutant prevention concentration assays were evaluated in four clarithromycin- and rifampin-susceptible (Cla(S)/Rif(S)) and two clarithromycin- and rifampin-resistant (Cla(R)/Rif(R)) R equi clinical strains. Results In this study evaluating a limited number of isolates, combinations of clarithromycin with doxycycline or minocycline, but not with rifampin, were generally synergistic in both Cla(S)/Rif(S) and Cla(R)/Rif(R) strains as determined by checkerboard testing. In time-kill assays, all antibiotics, both alone and in combination, reduced viable Cla(S)/Rif(S) R equi by more than 3 log10 at 24 hours compared with control cultures without antibiotics. Combinations of clarithromycin with doxycycline, minocycline or rifampin induced significantly lower MPC values compared with the individual antimicrobials alone for all Cla(S)/Rif(S) R equi strains, resulting in a narrower mutant selection window (MSW). However, clarithromycin/rifampin combination did not markedly decrease MPCs of the individual antimicrobials in Cla(R)/Rif(R) R equi isolates, and the observed decrease in MPCs for doxycycline or minocycline did not generally differ when combined with clarithromycin. Main limitations The number of analysed R equi isolates was limited. In vitro outcomes require clinical confirmation. Conclusions Dual therapy combinations consisting of clarithromycin with doxycycline or minocycline merit consideration as a treatment protocol against R equi in foals due to in vitro synergy. These combination therapies may also minimise the emergence of antimicrobial resistance in cases of rhodococcosis.